The Green party’s dramatic capture of Gorton and Denton, supposedly one of Labour’s safest parliamentary constituencies, offers yet more evidence of the fragmentation of British politics.The Green candidate, 34-year-old plumber Hannah Spencer, won 40.69% of the vote, a notable 12 points ahead of Reform UK’s Matt Goodwin (28.73%). Labour candidate Angeliki Stogia came third, with 25.44% of the vote.In terms of size of majority toppled, this was the sixth-worst byelection defeat ever for Labour. Gorton had been Labour for more than 90 years. In what is now Greater Manchester, Labour has had to defend 20 seats at byelections since the second world war, and has been successful in 16 cases. Although Labour might dismiss a byelection defeat as a mid-term blip, this is a government which has failed to enjoy a honeymoon period, led by a prime minister who has plumbed new depths in popularity ratings. It is also worth noting that turnout on Thursday was identical to that at the general election.For Keir Starmer, it was a truly awful result. But Labour really lost this byelection over a month ago, when its national executive committee (NEC) blocked the candidature of Andy Burnham, the mayor of Greater Manchester and believed by many to be the one person who might have held the seat for the party.Starmer spoke against Burnham standing and his view held sway at that NEC meeting by eight votes to one. The one vote in favour of Burnham standing came from a Manchester MP, Lucy Powell, who was elected Labour’s deputy leader last year after being mysteriously sacked from the cabinet by Starmer.The prime minister had good reasons for his stance. An early exit by Burnham from the mayoralty would trigger a difficult byelection across the region. But the overarching reason for blockage appeared to be Starmer’s personal political security. Popular among Labour members and perhaps the one rival to Starmer around which the parliamentary party could coalesce, Burnham might have offered a potential leadership challenge. He is also highly popular in Greater Manchester, averaging two-thirds of the vote in the three mayoralty contests he has fought. The newly elected Green MP Spencer trailed Burnham by a huge 375,000 votes in the most recent mayoral election in 2024.Clearly, this poor result increases the pressure on the prime minister, but two things remain in his favour. First, Labour MPs may find it difficult to unite behind a clear challenger. Entry barriers are high; 80 MPs need to support the person prepared to raise their head above the parapet. Second, the economy is showing signs of improvement, which might eventually stem the flow to the Greens on the left. On the right, the exodus towards Reform may be slowed by the decline in net migration.Yet things will get worse before they might get better for Labour. The Scottish parliament, Welsh senedd and English local elections are a mere 69 days away, and offer a bleak vista of large seat losses. Labour’s control of the senedd seems sure to end and the party has to defend the bulk of council seats being contested.The end of two-party politics?The Gorton and Denton result confirmed the death of old loyalties in British politics. Given the existence of four-party politics in Scotland and Wales and the electoral significance of the Liberal Democrats in England, the two-party duopoly has long been gone, perhaps never to return. Politics has never been as fragmented across parties.For the first time in England, Labour finds itself challenged by a significant party of the left, while Reform on the right challenges both Labour and the Conservatives. That the right vote is splintered offers some succour to Labour. An even split between Reform and the Conservatives could allow Labour to win again at the next general election, with an even more pitifully low percentage share of the vote than the one in 2024 which nonetheless yielded two-thirds of the Westminster seats. This fragmentation may widen voter choice, but not all is healthy. This was at times a toxic byelection. The Greens argued it was possible to be jointly concerned with Gaza and Gorton. They were, however, accused of sectarianism, for example by by issuing Urdu-language leaflets and a campaign video showing Starmer greeting the Indian prime minister, Hindu nationalist Modi, to appeal to Muslim voters.Reform, on the other hand, has been accused of racism in targeting the white vote and showing scant regard for the large Muslim minority within the constituency. Its candidate, former academic Matt Goodwin, was already controversial for his views questioning whether non-white people born in the UK could be classed as British.Meanwhile, the first-past-the-post voting system struggles to deal with the reality of modern multiparty politics, with abject disproportionality between vote shares and levels of representation. But that fragmentation increasingly seems permanent.Jonathan Tonge does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.